My flippant response would be everything I know... because if I learned everything sooner, I could learn even more than I know now.

But my serious answer would be that taking care of your hair is important... at least if you want to keep it. When I was younger, I had really long hair and I didn't take care of it very well. Used to wear it in tight ponytails when I went to bed and in hindsight, I answer washed it way too much (1-2 times a day).

While I think genetics has had a hand in my hairline, I also think that it could be in better shape if I looked after my hair a bit better when I was younger.

Oh well... at least I look okay with my head shaved :)